1. Field of the Invention
The invention relates to a method and apparatus for reading magnetically coded data.
2. Description of the Prior Art
The automatic processing of data carried by documents requires the data to be written in a form in which it can easily be detected and recognized by the processing machine. Magnetic coding of the data by the application of magnetic elements to the documents has the advantage of concentrating the data at a high density in an analog or digital representation, of making it easier both to write and to read, and of involving relatively simple, low-cost materials and equipment.
Magnetic coding of this kind is widely used at present, in particular by organizations which deal with money, to record on the individual documents (such as checks) which are made available to customers to perform predetermined operations; the usual details such as the code numbers of a bank, of the branch, of the drawer's account and of the document.
A plurality of analog or digital codes have been adopted by organizations which deal with money. In Europe, and particularly in the Latin countries, the most widely used code is that known by the abbreviation CMC7 (7 element coded magnetic character). This code provides a way of representing alphabetic and numerical data which has the advantage of being readable both by a machine and by an untrained person. Each group of seven elements are arranged vertically parallel to one another on the document, as shown, for example, in FIG. 2A herein, and suggests the shape of the character in question. However, for the machine, the seven elements making up the character are separated from one another by long (b) or short (a) intervals which are predetermined by the character concerned. In addition, each character is separated from its neighbor by an interval different from the aforenoted (a) and (b) intervals, which intervals are called a very long (c) interval or an inter-character interval.
As an example, the figure "O" is defined by reading in succession from left to right, two short intervals, two long intervals, and two short intervals. The figure "1" is defined by one long interval, three short intervals, one long interval and one short interval. The letter "A" is defined by one short interval, one long interval, and four short intervals, and the letter "Z" by two short intervals, two long intervals, and two short intervals. The CMC7 code is well known and will not be described in detail herein.
The decoding devices process information coming from magnetic readers, and to enable them to do this, the elements (also called bars) of the characters are printed on the documents in an ink which can be permanently magnetized, while the readers are preceded by a magnetizing device which is designed to saturate the elements magnetically. The elements are thus capable of generating electrical pulses in the reader and the decoding devices then process these pulses digitally to reconstitute the data in analog form.
Another digital code, which tends to find favor for more use at present on credit cards, is that generally referred to as the "dual frequency code" or "Aiken code". This is a purely magnetic digital code formed by a continuous succession of equal magnetized cells. In this succession, the cells representing zero bits are magnetized in their entirety in the direction in which the cells succeed one another. Cells representing one bits are magnetized in halves in two opposing senses in the said direction. In addition, each cell is differentiated from the adjoining cell by a reversal of its sense of magnetization.
The reading of data written in this code consists, on the one hand, of detecting the magnetization of each cell by generating, in a reader, alternating electrical pulses whose length varies as a function of the items of binary data defined by the cells, and on the other hand of making measurements of these lengths to reconstitute the corresponding data.
The English speaking countries have, for their part, developed an entirely analog code called the E13B code. This code is the one most widely used throughout the world. In this code, each character is written on the document in a predetermined form which is well suited to optical decoding. However, an optical reader is a relatively complex and expensive instrument compared with a magnetic reader. However, when written in a magnetizable ink and then magnetized before being read, characters so coded are capable of producing in a magnetic reader electric signals specific to each character.
Magnetic decoding is currently performed on one of the following two principles.
The first method of reading consists in passing the document in front of a magnetic reader, which reacts to variations in magnetic flux to produce a corresponding electrical signal. The detected signal is thus proportional to the differential coefficient of the variation in the magnetic induction field with respect to time, that is to say, to the relative speed of passage of the document. Thus, correct signals can be obtained from readers of this kind only if the speed of passage is relatively high.
The second method of reading has the advantage of being independent of the speed of movement of the documents past the reader. In this case the readers have magnetoresistors as their detecting elements. These latter are electrical resistors which are deposited on a substrate of insulating material in the form of thin films or layers of extremely small thickness (a few hundreths of an Angstrom to a few microns) and whose resistance R varies by an amount .DELTA.R proportional to the strength of the magnetic field which they receive. In this way, if they carry a constant current I, the change in resistance .DELTA.R will be reflected by a change in voltage .DELTA.V=I. .DELTA.R. The ratio .DELTA.R/R is called the coefficient of magnetoresistance.
Magnetoresistors in current use are formed from an anisotropic magnetic material. In such a material, two axes are defined, namely, the axis of easy magnetization and the axis of hard magnetization. The magnetic permeability .mu. of the magnetic material is at a maximum in the direction of the axis of hard magnetization and at a minimum in the direction of the axis of easy magnetization.
Any magnetic field H.sub.e generated externally to the magnetoresistor and applied thereto creates in it a demagnetizing field H.sub.de which tends to counteract the field H.sub.e. The magnetoresistor reacts to the magnetic exciting field H, whose modulus is that of the sum (/H/=/H.sub.e +H.sub.de /) of the corresponding fields H.sub.e and H.sub.de.
A magnetoresistive material is also characterized by a specific magnetoresistance curve. This defines the changes in the resistance of the material as a function of the magnetic exciting fields H to which the material is sensitive. The curve is normally in the shape of a bell which is symmetrical with reference to the sign of the exciting fields, that is to say, about the axes carrying the values of resistance.
Because of this symmetry, alternating changes in the exciting field H about the value zero result in non-alternating changes in the resistance of the material. To overcome this disadvantage, the magnetoresistor is polarized by means of an external magnetic polarizing field H.sub.p which, together with the corresponding demagnetizing magnetic field H.sub.dp forms a magnetic exciting field H.sub.t =H.sub.p +H.sub.dp termed the translatory field. The effect of this field is in fact to shift the curve for magnetoresistance along the axis of the magnetic exciting filds H. Detection thus takes place about a central point situated on one side of the curve.
Corresponding to this translatory shift is an angle of polarization .crclbar. between the magnetization vector of the magnetoresistor and its axis of easy magnetization. This angle varies with the value of the modulus of the polarizing field H.sub.p between 0.degree. and 90.degree..
It appeared obvious and natural to make the value of the angle of polarization approximately 45.degree., on the one hand because of the symmetry, which was favorable to the detection of an alternating signal, and on the other hand because of the high sensitivity to external magnetic fields which the magnetoresistive material has with this value. It is, in fact, approximately at this point that the sign of the curvature of each side of the magnetoresistance curve changes. An example of magnetic detection with 45.degree. polarization is described in U.S. Pat. No. 3,848,217.
Detection of this kind has been shown to have the following disadvantages. Firstly, when the magnetic exciting field to be detected (not the magnetic polarizing field), which is also referred to by the name "magnetic signal field", has a modulus greater than that of the translatory field H.sub.t, distortion exists in the voltage signal taken from the terminals of the magnetoresistor. Secondly, a reduction in the resolution of the detection is found. Resolution is the ratio between the peak-to-peak amplitudes of the high frequency signals relative to the low frequency signals. The distortion in the detected signals, combined with the reduction in resolution, reduces the reliability with which the data is read. Given that the detecting threshold level for the electrical circuits which process the signals from the magnetoresistor must be reduced by an appropriate amount, the decoding circuits are made more susceptible to magnetic interference fields.